Hi Mark

In one word, a lot.

Kontakt Player which comes with GPO is only a player. Kontakt 2 could be compared with Tascam's GigaStudio but I find the ease of working with it outstanding. What you can do with it, layer sounds, HD streaming, surround, effects, modeling.
You can create new sounds from existing samples and all this in an easy and simple way.

In fact it is too much to sum up here so visit Native Instruments

If you are serious about sampling than Kontakt is a must.
With GPO Studio you can only play the GPO-samples and that's it.

Kontakt 2 full version cost $499, but if you are a registered owner of Kontakt player than an upgrade to the full version is $350.
With it comes a huge 15 Gb samplebank. (even VSL is included, at least a small version).

It isn't cheap but worth every penny.

Cheers........Erwin